1. Field of the Invention
The present invention relates to an optical pickup, and, more particularly, to an optical pickup including a front photo-detector to detect power of light incident on an objective lens.
2. Description of the Related Art
FIG. 1 schematically shows the structure of an optical pickup used in an optical disc drive. Referring to FIG. 1, the optical pickup includes a light source 1, a grating 2, a collimating lens 3, an objective lens 9, and a main photo-detector 7. The light source 1 emits a light beam having a predetermined wavelength. The grating 2 diverges 0th-order and ±1st-order beams from the light beam. The collimating lens 3 transforms the divergent beams emitted from the light source 1 into parallel beams. The objective lens 9 enables the light beam to be focused as a light spot on a recording surface of an optical disc 10. The main photo-detector 7 receives a light beam reflected from the recoding surface of the optical disc 10 to detect an information signal and/or an error signal.
Reference numeral 4 denotes a beam splitter that enables the light beam emitted from the light source 1 to be incident on the objective lens 9, and that guides the light beam, which has been reflected from the optical disc 10 and passed through the objective lens 9, toward the main photo-detector 7. A concave lens 6 may be interposed between the beam splitter 4 and the main photo-detector 7 to prevent astigmatism.
A light beam having a predetermined power level should be incident on the optical disc 10 so as to record information and/or reproduce information from the optical disc 10. As shown in FIG. 1, the optical pickup further includes a beam splitter 5 and a front photo-detector (FPD) 8. The beam splitter 5 splits the light beam, which has passed through the beam splitter 4, into two beams so as to be incident on the objective lens 9 and the FPD 8, respectively. The FPD 8 detects power from the incident beam portion. When the detected light power is lower or higher than a predetermined level, the power of the light source 1 is adjusted so that a light beam having a predetermined power level is incident on the optical disc 10.
As described above, optical elements of an optical system are installed on a pickup base which moves in a radial direction of an optical disc. When an optical pickup is manufactured, it is necessary to adjust locations of the optical elements and angles among them when installing the optical elements. In other words, the installation locations and angles for the optical elements must be precisely adjusted to focus the light beam emitted from the light source 1 as a light spot of a predetermined size on a predetermined portion of the optical disc 10.
The FPD 8 should always be located relative to the light source 1 to precisely adjust the light power of the light source 1 to a predetermined level, so that the light power detected by the FPD 8 is proportional to the light power of the light source 1. However, as described above, when the locations of the light source 1 and the FPD 8 are separately adjusted, the locations of light source 1 and the FPD 8 may vary when an optical pickup is manufactured. In this case, the relationship between the light power of the light source 1 and the light power detected by the FPD 8 may vary when an optical pickup is manufactured. Thus, it is quite difficult to adjust the light power of the light source 1 to a predetermined level. Accordingly, whenever the location and angle of the light source 1 are adjusted, the location and angel of the FPD 8 must be adjusted. As a result, the cost of manufacturing an optical pickup may increase.